Tiny swimmers mix it up in ocean ‘hotspots’

"There are local hotspots where this mixing can be comparable to the turbulent mixing in the mid-ocean," says Arezoo Ardekani. "The mixing induced by horizontally swimming organisms is one hundred times weaker than the contribution of vertically swimming organisms." (Credit: Macroscopic Solutions/Flickr)

Small marine organisms swimming in concentrated “hotspots” likely contribute to the mixing of water needed to distribute nutrients for ocean species.

The ocean consists of a top nutrient-depleted layer that is well mixed by turbulence created primarily by wind, tides, and other mechanisms, and a bottom unmixed nutrient-rich layer. In between is a layer called the pycnocline marked by a sharp change in density.

“We are trying to learn about the potential contribution of marine organisms to the mixing processes that transport water from this nutrient-rich bottom region through the pycnoline layer and to the nutrient-depleted region,” says Arezoo Ardekani, assistant professor of mechanical engineering at Purdue University.

Ocean mixing is critical for distributing nutrients to the nutrient-depleted layer. “Some marine organisms move vertically between the top and bottom layers during the day and night through a process called diel vertical migration,” Ardekani says.

While it is known that wind and waves are the primary cause of ocean mixing, one unknown factor is whether swimming organisms contribute to this vital mixing. Previous research has suggested the organisms transport volumes of fluid and nutrients with them, a concept known as Darwinian drift. However, some scientists argue that the drifted fluid can re-stratify in marine environments and may not cause mixing.

For the new study, published in the journal Scientific Reports, researchers used high-fidelity numerical simulation of a “simplified swimming model” to determine whether the small ocean organisms such as zooplankton swimming in concentrated hotspots contribute to the mixing.

This animation shows how “hotspots” of small swimming organisms might contribute to the mixing of ocean waters. (Credit: Shiyan Wang and Arezoo Ardekani)/Purdue University

“So there are local hotspots where this mixing can be comparable to the turbulent mixing in the mid-ocean,” Ardekani says. “The mixing induced by horizontally swimming organisms is one hundred times weaker than the contribution of vertically swimming organisms.”

The hotspots may be especially important in the mid-ocean, where the majority of energy from waves and wind is dissipated prior to contributing to mixing.

The research was carried out with high-performance computing clusters operated by Information Technology at Purdue and also the Center for Research Computing at the University of Notre Dame.